I note that you bender is somewhat different to mine but the principles should be the same.
Ok some pics of mine. As Philip says you are clearly missing the rollers, these look much like a ball bearing and there should be 6 as you can see from my pice below.
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Above with a length of rail in to show how it fits, but Inthink you may have already worked that out.
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Above without some rail, felt line shows how rail sits in bender.
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Above showing the top, I have given my self a reminder in felt to show how the bender works as it can be years between each time I use it! Noting the scale 0 is where you start, anti clockwise turn goes to less press with the rollers makes fitting the rails easier and clockwise moves the scale up thus pressing the rollers harder onto the rail and thus curving it.